X-ray generating equipment

ABSTRACT

An X-ray generating apparatus for generating X-rays by irradiating a target with an electron beam. Wherein the apparatus includes a vibration applying means for vibrating the target in directions parallel to a surface thereof. A colliding spot of the electron beam is movable on the target while maintaining an X-ray focus in the same position on the electron beam without fluctuating the X-ray focal position. This enlarges an actual area of electron collision on the target to disperse the generated heat, thereby to suppress a local temperature rise of the target due to the electron collision. The X-ray generating apparatus is compact, and has a long life and a high X-ray intensity.

TECHNICAL FIELD

This invention relates to an X-ray generating apparatus for anon-destructive X-ray inspecting system or X-ray analyzing system. Oneof X-ray generating apparatus is a X-ray tube comprising a cathode withan electron-emissive element and an anode with an anode target platewhich are accommodated in an vacuum envelope. More particularly, theinvention relates to an apparatus having a very small X-ray source sizedin the order of microns to obtain fluoroscopic images of a minuteobject.

BACKGROUND ART

X-ray generating apparatus of the type noted above are disclosed inJapanese Unexamined Patent Publications 2002-25484, 2001-273860 and2000-306533, for example.

In these apparatus, electrons (Sa [A]) are emitted from an electronsource maintained at a high negative potential (−Sv [V]) in a vacuum.Secondly, the electrons are accelerated by a potential differencebetween the electron source and ground potential 0V. Thirdly,accelerated electrons are converged to a diameter of 20 to 0.1 μm withan electron lens. Finally, the converged electrons collide against asolid target formed of metal (e.g. tungsten (W), molybdenum (Mo) orcopper (Cu)), thereby realizing an X-ray source sized in the order ofmicrons. A maximum energy of generated X-rays is Sv [keV].

An especially high-resolution apparatus among these apparatus is calleda transmission X-ray generating apparatus or a transmission X-ray tube.Such an apparatus, for example, has a target with a film thickness ofabout 5 μm formed on a thin aluminum holder (e.g. 0.5 mm thick plate).X-rays generated at the target are transmitted through the holder, inthe direction of incident electron beam, and transmitted X-rays areutilized in the atmosphere. The above holder is called a vacuum window,which is used because the thin target in film form is not strong enoughto withstand atmospheric pressure. The vacuum window is clamped tightand fixed to a vacuum vessel by an O-ring or the like. This fixingportion is the center of a forward end of an electron lens, and has anevacuated path with a diameter of about 10 mm for converging and passingthe electron beam.

In such a transmission X-ray generating apparatus, the target isdisposed very close to the electron lens. As for the primary reason,thereby reducing the influence of aberration of the electron lens, andalso the diameter of electron convergence is minimized. Thus a minimumX-ray focus is obtained, and high-resolution X-ray fluoroscopic imagesare realized. As for another reason, thereby the inspection object isclose to the X-ray focus, and thus high magnification images obtain.Such an transmission X-ray tube is used in an inspection apparatus forsearching for minute defects in an inspection object. These inspectingoperations will sometimes take several hours per object. Theconventional apparatus constructed as described above has the followingdrawbacks.

When accelerated electrons (electrical power Sa·Sv [W]) collide with thetarget, a large part of the electrical power changes into heat, therebyresulting in an X-ray generating efficiency of 1% or less. The heatgenerated by the electron collision raises the temperature of anelectron-colliding portion of the target. Consequently, the temperatureraise evaporates the target material and causes various problems.

Thus, the transmission X-ray generating apparatus is halted at the endof target life. The vacuum window clamped to the vacuum vessel isloosened and turned or changed, so that the electron collision portionis replaced to a new target surface. Subsequently the operation of theapparatus is resumed. This causes a problem that X-rays cannot begenerated continuously over a long period of time, or a problem oflowering the operating ratio of the X-ray generating apparatus.Particularly where a large object is inspected, the apparatus isoperated with an increased load power in order to increase X-rayintensity. In such a case, the life of the target is short and the X-raygenerating apparatus must be halted frequently. Further, there is alimit to the X-ray intensity that can be outputted. Since the microfocusX-ray tube is relatively dark, its working throughput cannot beincreased.

A method of trial calculations of a target life from electron beam powerand a beam diameter is described hereunder.

When an absorbed electric power (Sv·Sa [W]) collides, within a circle ofdiameter s [μm], with a surface of semi-infinite solid of thermalconductivity K [W/cm° C.], the steady state temperature rise ΔT [° C.]is expressed as follows (reference: Junzo Ishikawa, “Charged ParticleBeam Engineering”, Corona Co., May 18, 2001, 1st edition, p145):ΔT[° C.]=2×10⁴·(Sv·Sa)/(πKs)  (1)

This equation (1) shows that the temperature rise is proportional to theelectrical power and is inversely proportional to the collision diameters. The equation shows also that the temperature rise depends on theelectrical power per diameter. Moreover, temperature rise ΔT isinversely proportional to the root of the collision area S, because thecollision area S is expressed as π(s/2)². For example, same electricalpower and four times area causes half temperature rise.

When the target is formed of tungsten (W), a trial calculation of ΔT isdone by using thermal conductivity K=0.9 [W/cm° C.] at the melting point(3,410° C.) of tungsten. And after the trial calculation, thetemperature of collision portion in the target at 27° C. (i.e. at roomtemperature) is given by a equation, T=300+ΔT [K].

Next, a trial calculation of an amount of evaporation d [kg/m² sec] ofthe solid at temperature T [K] is done by the following Langmuirequation (2):d=4.37×10⁻³ ·P√(M/T)  (2)

In this equation, M is the atomic weight of a solid material, and thatof tungsten is M=183.8. P[Pa] is the vapor pressure of the solid at thetemperature T[K] and is derived from the following equation (3):logP=−A/T+B+C logT−DT+2.125  (3)where constants A=44000, B=8.76, C=5 and D=0.

A trial calculation of an amount of evaporation (thickness) per unittime [μm/time] is done by changing the unit of the above amount ofevaporation d, thereby dividing by the density of tungsten (19.3[g/cm²]). Further respecting for a small X-ray focus, the target life isregarded as a time evaporating a thickness corresponding to thecollision diameter s.

Results of trial calculations are shown in FIG. 1 under various electronbeam conditions and various problems are discussed hereinafter.

Problem 1

“An operating time loss is caused by the target life.”

Load condition No. 1 is an example of ordinary use load of themicrofocus X-ray tube. An electron beam power 0.32 W collides with acollision diameter s=1 μm, as a result of a calculation, the temperatureof the colliding portion is 2,576K and the life is 142 hours.

In this case, the apparatus is stopped every 142 hours for maintenancework, the vacuum window is loosened and is turned to receive theelectron beam on a new target surface. Once loosening the vacuum windowbreaks the vacuum, and the envelope must be evacuated again for abouttwo hour. Then the operation is resumed. Thus, X-rays cannot begenerated for about two hours, and hence there is a problem of loweringthe operating ratio of the apparatus. Consequently maintenance work hasto be done for two hours once a week, and this operating ratio is142/(142+2)=99% for assuming a continuous operation. In some case thelife will be extended by lowering the power, however reducing X-rayintensity and requiring a longer time for fluoroscopy, thereby workingthroughput will reduce.

Problem 2

“There is an upper limit to X-ray intensity, and no improvement inworking throughput.”

Load condition No. 2 is an example in which X-ray intensity is slightlyhigher than the loading condition No. 1. The current is increases by 9%with the same acceleration voltage, and also the electron beam powerincreases by 9% from 0.32 W to 0.35 W. Thus, X-ray intensity increasesby 9% and working throughput also by 9%. However, as a result of acalculation, the temperature of the colliding portion is 2,790K and thelife is calculated to be seven hours. In this case, the mere 9%increases in X-ray intensity results to stop the apparatus every sevenhours for maintenance work. The operating ratio of the apparatus fallsoff to 7/(7+2)=78%.

Load conditions No. 3 and No. 4 are examples where X-ray intensity isabout three times that of load condition No. 1. As a result of the trialcalculations, the temperature of the colliding portion exceeds thefusing point (about 3,680K) and boiling point (about 6,200K) oftungsten. Since the target material evaporates quickly, these conditionsare impracticable. If X-ray intensity were increased by three times,working throughput would be three times higher since the time requiredfor generating the same X-ray dosage would be one third. Consequently,there is a limit to load power and an upper limit to X-ray intensity,hence working throughput cannot be improved.

Problem 3

“The tube is darkened by minute focusing.”

Temperature rise ΔT is dependent on the electron beam power per diameteras expressed by equation (1). Therefore, when the electron beam isnarrowed down to reduce the collision diameter, the the electron beampower must also be reduced. Assume, for example, a case where thecollision diameter s=0.1 μm to secure a minute X-ray focus for higherresolution. Since power must be reduced to one tenth in order to obtainthe same evaporation rate as in load condition No. 1, X-ray intensityalso becomes one tenth and working throughput one tenth. Moreover, sincethe life is determined by “Further respecting for a small X-ray focus,the target life is regarded as a time evaporating a thicknesscorresponding to the collision diameter s”, the evaporating thickness tothe end of life is one tenth, and life is reduced to one tenth, i.e.14.2 hours. The operating ratio of the apparatus decrease to14.2/(14.2+2)=88%. Such minute focusing is needed in order to cope withthe micro-fabrication of integrated circuits in the semiconductor fieldtoday, and therefore is all the more problematic. Load condition No. 5is a desirable example in which the collision diameter s=0.1 μm and theelectrical power is set to 0.24 W which is 75% of the load conditionNo. 1. As a result of the trial calculations, the temperature of thecolliding portion is 1,7371K, and the quick evaporation makes thiscondition impracticable.

Problem 4

“Caution is needed because of delicate changes in focus shape.”

When X-ray irradiation is carried out continuously for 142 hours withthe load condition No. 1 in FIG. 1, the target becomes thin as a resultof the 1 μm evaporation. During this evaporation, the shape of thetarget surface struck by the electron beam varies, and the shape andposition of the X-ray focus undergo delicate changes. Since a microfocusX-ray apparatus is required to keep high spatial resolution, a fineadjustment of the electron beam is needed even within the lifetime.Therefore, this reduces the operating ratio of the apparatus. Moreover,it should be noted that the life shown in FIG. 1 is tentative and notabsolute.

Problem 5

“A thick target unnecessarily absorbs X-rays.”

In order to provide a similar X-ray intensity during a life, the targetshould have a thickness at least equal to a sum of a maximum depth ofelectron penetration and a thickness corresponding to the target life.Also in order to withstand power increases due to voltage variations orthe like, the target usually is formed somewhat thick.

For example, accelerated electrons with an energy of 40 keV at the timeof a 40 kV tube voltage collide with the tungsten target and enter thetarget by a maximum depth of 2.6 μm while generating X-rays of 40 keV orless. Thus, for the 40 kV tube voltage and 1 μm collision diameter, atarget thickness of at least 3.6 μm is needed, and a thickness of about5 μm is adopted to allow for a margin.

However, since the maximum depth of the X-ray generating region is 2.6μm, only the X-rays not absorbed by the remaining 2.4 μm of the targetthickness of 5 μm is used as transmitted X-rays. This constitutes a lowutilization rate of the generated X-rays. Where, for example, X-rays of20 KeV pass through the tungsten of 2.4 μm, only 80% is transmitted.Thus, X-ray intensity is low and the working throughput falls off to80%.

Problem 6

“A rotating anode X-ray tube is incapable of high resolution.”

To solve the problem caused by the heat of the target, an X-raygenerating apparatus of millimeter-size focus for medical use employsthe rotating anode type. However, rotational accuracy is insufficientwith a bearing (ball bearing) used for rotation, and the anode target isnot rotated with high accuracy, then the X-ray focus is blurred.Therefore the rotating target is difficult to apply particularly to themicrofocus X-ray generating apparatus having an X-ray focal size in theorder of microns. The above problem is discussed more particularlyhereinafter.

The rotating anode X-ray tube has an X-ray focal size in the order of0.2 to 1 mm, and has a vacuum vessel, an electron source, an anode disk,a rotating bearing and a motor formed as an integrated unit. But themotor is spaced from the electron beam, because the motor generating anelectromagnetic force deflects the electron beam unnecessarily. Thus,the rotating anode X-ray tube tends to be large. Further, a ball bearingis employed as the rotating part and has an inside diameter of 6 to 10mm, an outside diameter of 10 to 30 mm or more, and a thickness of 2.5to 10 mm or more. The highest accuracy class of ball bearings in thisrange of sizes is specified in Class 2 of the Japanese IndustrialStandards, and the axial deflection accuracy and radial deflectionaccuracy of the inner ring are as much as a maximum of 1.5 μm. Since theX-ray tube is used in severe conditions of high vacuum, high temperatureand high speed, a special lubricating system is used. The degree ofvacuum inside the X-ray tube, for example, has to be 0.13 mPa (10⁻⁶Torr) or less. The bearing is operable in the temperature range of 200to 500° C. due to the generating heat of the anode, and a high-speedrotation in the order of 3,000 to 10,000 rpm (50 to 167 cyc/sec) is alsorequired. In order to satisfy such severe conditions, the X-ray tubeemploys a very special bearing using a thin coating of soft metal assolid lubricant. However, since the life of the solid lubricant isshort, the life of the rotating anode X-ray tube also has a life of onlyseveral hundred hours.

The microfocus X-ray tube has a lower load power than the X-ray tube formedical, therefore the target holder does not reach such a hightemperature. However, bearing steel has a coefficient of linear thermalexpansion in the order of 12.5×10⁻⁶ (1/° C.), and a temperature rise ofonly 20° C. lowers its rotational accuracy with the inside diameterexpansion of 1.5 to 2.5 μm. A temperature rise of about 20° C. easilyoccurs with a change in a room temperature or with a heat generated byrotation friction. Combined with the rotational accuracy specified inClass 2 of the JIS, a rotational accuracy of 3 μm or less is unwarrantedand impracticable. Further, the rotating anode disk have a diameter of10 mm or larger because of the outside diameter of the bearing, and thewhole waviness of the target surface, since tungsten is extremely hardand difficult to shape, varies the X-ray focal position by about 10 μm.Accuracy of this level is not problematic with the medical X-ray tubewhose X-ray focal size is about 0.2 to 1 mm. However, with themicrofocus X-ray tube whose X-ray focal size is in the order of microns,focal size variations and focal position shift in the electron beamdirections make the application of the rotating anode type difficult.

The bearing is at least five times thicker than the transmitted X-raytype vacuum window which is about 0.5 mm thick, whereby the rotatinganode type has to be large. The rotating anode requires a vacuum windowas an essential component for acquiring X-rays. That is, the rotatinganode and an object under inspection cannot be brought close to eachother, and it is accordingly difficult to increase geometricmagnification. Even if a high-accuracy ball bearing is developed, itwill be difficult to obtain high-resolution X-ray fluoroscopic images.

DISCLOSURE OF THE INVENTION

This invention has been made having regard to the state of the art notedabove, and its object is to provide an X-ray generating apparatus withhigh resolution and compactness, for extending the life of a target,increasing the operating ratio of the apparatus, extending a time ofcontinuously generating X-rays, and improving X-ray intensity.

The above object is fulfilled, according to this invention, by anapparatus for generating X-rays by irradiating a target with an electronbeam, comprising a vibration applying means for vibrating the target indirections parallel to a surface thereof.

The vibration applying means vibrates the target in directions parallelto the surface thereof. Whether the apparatus is the transmission typeor reflection type, a colliding spot of the electron beam is moved onthe target surface while maintaining an X-ray focus in the same positionon the optical axis of the electron beam without fluctuating the X-rayfocal position. This enlarges an actual area of electron collision,disperses the generating heat, thereby suppress a local temperature risedue to the electron collision. Thus, evaporation of the target issuppressed. As a result, the target is given an extended life, toincrease the operating ratio of the apparatus resulting from changingand adjustment of the target. Moreover, X-ray intensity also increase.

The vibration in this invention is a shaking motion in substantiallyfixed cycles, having functions and effects not acquired simply byrotating the target.

That is, by rotation, the electron beam will repeatedly move along thesame track on the target. By vibration, on the other hand, the electronbeam is not only moved on the same track, but, for example, is vibratedto describe the same track in a first area on the target, and after apredetermined time the electron beam is moved to a second area andvibrated to describe the same track therein. With such vibration, theelectron beam can be moved on different tracks on the target, toincrease a more actual area of electron collision. Compared with therotation type describing a fixed track, thus using only part of thetarget, the vibration type can make effective use of the entire surfaceof the target by setting various tracks of the electron beam on thetarget surface.

Conversely, the area of the target is reduced so that the target issmall and lightweight, and that the vibration applying device also isreduced in size. Thus, the X-ray focus and an object under inspectionare brought close to each other to obtain high-resolution X-rayfluoroscopic images with geometrically increased magnification.

The vibration herein has a wide range of cycles including every severalmonths, several weeks, several days, several hours, several Hz, severalkHz and several MHz.

Preferably, the vibration applying means is arranged to vibrate thetarget so that the electron beam has a colliding spot describing, on thetarget, a linear track, a circular track, or a two-dimensional shapeincluding zigzag and rectangular shapes.

By vibrating the target so that the electron beam describes, on thetarget, a one-dimensional shape such as circular arc or a straight line,or a two-dimensional shape such as a zigzag, rectangular or squareshape, vibration applying means is effected relatively easily andenlarge the effective area of electron collision. A two-dimensionaltrack in particular allows the target to be especially small and thevibration applying device also to be small.

The apparatus according to this invention, preferably, further comprisesa vibration controller for controlling the vibration applying means. Avibration is controlled in one of a tube voltage, a tube current, anelectron beam diameter, and a temperature measured adjacent a spot ofelectron beam collision.

A temperature rise of the target is proportional to a tube voltage and atube current, and inversely proportional to a diameter of electron beamcollision. Thus, suitable vibration is applied by controlling the holderof the target based on these factors.

Preferably, the vibration controller is arranged to control thevibration amplitude more than the electron beam diameter.

By vibrating the target with an amplitude at least corresponding to theelectron beam diameter, no part of the target is constantly irradiatedby the electron beam, thereby a temperature rise is uniform. It is stillmore desirable to control the vibration to have an amplitude at leasttwice the electron beam diameter. Furthermore, increasing vibrationamplitude decreases the temperature rise of the part of electron beamcollision. The vibration amplitude is arranged in proportion to theelectron beam power or inversely proportion to the electron beamdiameter.

Preferably, the vibration controller is arranged to make a frequency ofvibration variable.

Increasing vibration frequency makes the uniform heat distribution ofthe area of electron beam collision, thereby suppresses a partialtemperature rise. The vibration frequency is arranged in proportion tothe electron beam power or inversely proportion to the electron beamdiameter.

The vibration applying means, preferably, includes a piezoelectricdevice.

A piezoelectric device does not produce a magnetic field, and thereforehas no adverse influence on the electron beam. A piezoelectric device isoperable at high speed and capable of minute displacement in the orderof microns. Thus, a piezoelectric device is well suited to the vibrationapplying device.

Preferably, the piezoelectric device is integrated with a holder andtarget to make a closed space.

A vacuum window is no longer needed for maintaining the target surfacein a vacuum, to simply the tube construction. Further, since the vacuumwindow is unnecessary, the distance between the X-ray focus andinspection object is minimized to enable high-resolution X-rayfluoroscopy with geometrically increased magnification.

Preferably, the apparatus according to the invention further comprisesflexures for attaching and supporting the holder.

The heat generated in the target is transfer away by heat conduction ofthe flexures, thereby suppressing a temperature rise of the entiretarget. Furthermore, since a deflection of the target in a directionalong the electron beam is reduced, the vibration is applied in thedirections parallel to the target surface and then suppresses deviationof the X-ray focus.

Preferably, the flexures are made by electrical discharge machining.

Electrical discharge machining assures high dimensional accuracy, andprocesses a thin metal flexure in the deep metal plate. Thus, theflexure have a high aspect ratio and is formed integrally on the holder.The flexures do not deflect the target surface from the collision spotof electron beam, and a precise vibration is possible. Furthermore itsheat conduction loss is minimized and the target temperature decreases.

Preferably, the target is vacuum-sealed by rubber elements or flexures.

Since vibration is applied to the holder, rubber elements or flexures,or both in combination, are used between the holder and the fixed vacuumvessel to absorb the vibration of the holder and target. In this way, avacuum seal is provided for the target surface. Thus, there is no needfor a separate vacuum window, to minimize a distance between the X-rayfocus and inspection object, and to enable high-resolution X-rayfluoroscopy with geometrically large magnification.

Preferably, the target has a thickness up to twice depth of electronspenetration calculated from a tube voltage and target materials.

The vibration applying means extends the target life, then it makes athick target unnecessary and realizes a minimum thickness target. Thisthickness approximately corresponds to depth of electrons penetrationcalculated from the tube voltage and target materials, but preferably atmost not exceeding twice the calculated depth. With this thickness, theunnecessary X-ray absorption is minimized to make efficient use ofgenerated X-rays. This is advantageous particularly when easilyabsorbable soft X-rays are used.

Preferably, the vibration controller is arranged to displace the targetwhen the electron beam applies a small load to the target.

When the electron beam applies a small load to the target so that thetarget lasts at least several hours or several days without beingvibrated, the vibration controller displaces the target only a distancecorresponding to at least several times the diameter of electron beamcollision, and then keeps the target still. Thus, the spot of electronbeam collision on the target is renewed only by displacement. The spotof electron beam collision is moved to a different position on thistarget within a much shorter time than on a fixed target, therebyeliminating a loss in operating time. The target will or will not bevibrated in each position.

Preferably, the vibration applying means is disposed in an opening inwhich the target is located.

Because aberration of electron lens is as small as close to the lens, anelectron beam convergent diameter is smaller near the lens. Thus theminimum X-ray focus is obtained when the target is in the opening of thelens. Furthermore, the vibration applying means locates in the opening,the compactness enables the X-ray focus and object to be close and raisephotographic magnification, thereby realizing X-ray fluoroscopy withhigh spatial resolution.

Preferably, the flexures are shaped thin in a direction of vibration ofthe target, and thick in a direction perpendicular to the direction ofvibration.

The flexures have a high aspect ratio and are driven in the direction ofvibration with a small force, but are difficult to move in the directionperpendicular to the direction of vibration. Thus, the target isvibrated with high precision without deflection in the direction alongthe electron beam.

Preferably, the target is disposed at an angle to the electron beam.

A reflection X-ray generating apparatus, as does a transmission X-rayapparatus, produces a similar thermal effect to realize a long life andhigh X-ray intensity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a table showing results of trial calculations made on variouselectron beam load conditions regarding the life of a target formed oftungsten;

FIG. 2 is a cross section showing an outline of an X-ray tube;

FIG. 3 is a block diagram showing an outline of an X-ray generatingsystem;

FIG. 4 is a schematic drawing showing a vibration track of an electronbeam on a target;

FIG. 5 is an enlarged schematic drawing showing areas of electron beamcollision;

FIG. 6 is a schematic drawing showing a different track of the electronbeam on the target;

FIG. 7 is a schematic drawing showing a further different track of theelectron beam on the target;

FIG. 8 is a schematic drawing showing a further different track of theelectron beam on the target;

FIG. 9 is a schematic drawing showing different tracks of the electronbeam on the target;

FIG. 10 shows a construction of a vibration unit, in which FIG. 10A is across section, and FIG. 10B is a front view;

FIG. 11 shows a different construction of the vibration unit, in whichFIG. 11A is a cross section, and FIG. 11B is a front view;

FIG. 12 shows a different construction of the vibration unit, in whichFIG. 12A is a cross section, and FIG. 12B is a front view;

FIG. 13 shows a different construction of the vibration unit, in whichFIG. 13A is a cross section, and FIG. 13B is a front view;

FIG. 14 shows a different construction of the vibration unit, in whichFIG. 14A is a cross section, and FIG. 14B is a front view;

FIG. 15 shows a construction of a cylindrical piezoelectric device, inwhich FIG. 15A is a perspective view, and FIG. 15B is a cross sectionshowing one mode of operation;

FIG. 16 shows a different construction of the vibration unit, in whichFIG. 16A is a cross section, and FIG. 16B is a front view;

FIG. 17 is a front view showing an outline construction using flexuresmanufactured by electrical discharge machining;

FIG. 18 is a cross section showing an outline construction usingflexures; and

FIG. 19 is a cross section showing an outline of a reflection X-raygenerating apparatus.

BEST MODE FOR CARRYING OUT THE INVENTION

Modes for solving the problem of the prior art include the following:

FIGS. 2 through 5 show one embodiment of this invention. FIG. 2 is across section showing a transmission X-ray tube. FIG. 3 is a blockdiagram showing an outline of an X-ray generating system. FIG. 4 is aschematic drawing showing vibration of an electron beam on a targetsurface. FIG. 5 is an enlarged schematic drawing showing areas ofelectron beam collision.

A transmission X-ray tube 1 has an electron gun 2 mounted in a vacuumvessel 3 for generating an electron beam B. The vacuum vessel 3 has anX-ray generating portion, shown in enlargement, opposed to the electrongun 2. The X-ray generating portion includes an end block 5 that is apart of pole pieces of an electron lens. The end block 5 has the bore 7that is formed centrally, and the bore 7 is a diameter of 10 mm or less.A target 9 is attached to a holder 11 fitted in the bore 7. The target 9is made from metal such as tungsten or molybdenum to generate X-rayswhen irradiated with an electron beam. A vacuum window 13 is disposedadjacent the holder 11. The vacuum window 13 is clamped by a mount ring17 screwed to the end block 5, and the vacuum window 13 serves as avacuum lock in combination with an O-ring 15 embedded around the bore 7.The holder 11 and vacuum window 13 are made from a material such asaluminum that transmits X-rays well. The wall thickness of vacuum window13 is in the order of 0.5 mm and is strong enough to maintain the vacuumagainst atmospheric pressure.

In the transmission X-ray tube 1, the electron beam B emitted from theelectron gun 2 is converged adjacent the electron lens pole piece of endblock 5 to irradiate the target 9. X-rays are generated from the target9 irradiated with the electron beam B, and are transmitted through theholder 11 and vacuum window 13 to emerge as irradiating X-rays 21. Withuse of an electron lens optical system, an electron converging positionis shiftable along a beam axis to vary a diameter of electron collisionon the target 9. It is thus possible to vary an X-ray focal size also.When the lens is adjusted so that the converging point is on the targetsurface, a minimum X-ray focus dependent on the aberration of theelectron lens is obtained. Although the electron convergence depends onthe type and arrangement of the electron lens, an electron convergencediameter in the order of nanometers can be obtained by an electronicoptical system such a SEM. Further, since an electron convergencediameter in the order of 5 to 100 μm is obtained with an electron gunonly having an electrostatic lens, a construction without a specialelectron lens is also conceivable. Furthermore, various tubeconstructions is considered depending on inspection objects andpurposes.

In this embodiment, the target 9 is vibrated by vibrating the holder 11with a vibration unit 23 disposed on the inner peripheral surface of thebore 7 in the end block 5. The vibration is applied in directionsparallel to the surface of the target 9 so that the X-ray focus positionis fixed during an electron beam irradiation. In this embodiment, theelectron beam is at right angles with the target surface, and thus thetarget 9 vibrates in perpendicular to the electron beam. However, thisperpendicular relationship is not essential in this invention.

The vibration unit 23, which corresponds to the vibration applying meansof this invention, is controlled by the vibration controller 25 shown inFIG. 3. The vibration controller 25 controls an amplitude, frequency andso on of vibration of the target. A tube voltage, tube current and so onapplied to the electron gun 2 are controlled by a high voltage generator27. The vibration controller 25 and high voltage generator 27 arecontrolled by a control unit 29 operable on instructions given by theoperator.

The vibration unit 23 vibrates the holder 11 and the target 9 linearly,so that a colliding spot of electron beam B reciprocates linearly on thesurface of the target 9. In this case the colliding spot is on a lineartrack as shown in FIG. 4, but the X-ray focus do not move.

As shown in FIG. 5, the vibration amplitude is desirably more than thediameter Ba of electron beam B. By controlling the vibration in thisway, no steady duplication of electron beam B occurs in time ofvibration, to provide an advantage of uniformly suppressing atemperature rise in areas of electron beam collision.

Next description shows that this embodiment improves the problems 1-4noted in the conventional example described hereinbefore. A plurality ofcharacteristic examples of the vibration applying means embodying thisinvention is described later herein. This order of description isadopted for the following reasons. Minute vibrations occur very easilyand infinite embodiments are possible, but are too numerous to describe.To describe certain specific embodiments could be misleading. Forexample, vibrations in the order of microns commonly occur in nature,and experience shows that a target could be vibrated by a slightpropagation of motor vibration. In the field of patent, vibrationisolating mechanisms is more meaningful than vibration mechanisms.Further, a specific basic component such as a ball bearing used in arotating mechanism is not conceivable for minute vibration as in thisinvention.

Trial calculations are executed to determine degrees of improvement inload conditions No. 1-4 in FIG. 1. When the electron beam B collideswith a conventional fixed target, the collision area S is π(0.5)²=0.79[μm²]. On the other hand, when the target 9 vibrates with a amplitude of5 μm as an example vibration in this invention, a total collision area Sof the electron beam is (π(0.5)²+1×5)=5.79 [μm²]. Therefore, thecollision area S becomes 5.79/0.79=7.3 times large, and S is convertedinto a circle of a diameter s 2.7 μm. Temperature rise ΔT derived fromequation (1) is 1/2.7 of the fixed target. The evaporation of tungstenderived from equations (2) and (3) reduces, and thus target life isextended. Results of the trial calculations of the life are shown in thecolumn “vibrating target” in FIG. 1. The degrees of improvement aredescribed hereunder.

Improvement regarding problem 1: “The operating time loss is eliminatedby a extremely long life.”

Load condition No. 1 is an example of ordinary use load of a microfocusX-ray tube. With this load condition No. 1, compared with the life of142 hours of the fixed target, the life according to this invention isimproved to 4.7×10²⁷ hours, which is regarded as an infinite life. Theoperating ratio of the apparatus is improved to 100%. The weekly twohours' maintenance is no longer necessary.

Improvement regarding problem 2: “X-ray intensity increases, and so doesworking throughput.”

Load condition No. 2 is an example in which X-ray intensity is slightlyhigher than in condition No. 1, and trial calculations are executed withthe power increased by 9% from 0.32 W to 0.35 W. With this loadcondition No. 2, compared with the life of seven hours of the fixedtarget, the life according to this invention is improved to 1.5×10²¹hours, which is regarded as an infinite life. The operating ratio of theapparatus is improved from 78% to 100%. The two hours' maintenancecarried out every seven hours is no longer necessary. The 9% increase inworking throughput due to the 9% increase in X-ray intensity over theload condition No. 1 for the fixed target is retained intact, to allowan inspecting operation of 9% increase.

Load condition No. 3 is an example where X-ray intensity is about 2.7times strong compared with the load condition No. 1. This condition isimpracticable with the fixed target. The life according to thisinvention is greatly improved to 189 hours. Compared with load conditionNo. 1 for the fixed target, the invention achieves an improvement inlife of 189 hours/142 hours=1.3 times, an improvement in X-ray intensityof 0.86 W/0.32 W=2.7 times, and an improvement in working throughput of2.7 times.

Load condition No. 4 is an example where X-ray intensity is about 3.1times that of load condition No. 1. This condition is impracticable withthe fixed target. The life according to this invention is no less than78 minutes. The invention provides an improvement in working throughputof 3.1 times over the fixed target under load condition No. 1.

The above improvements in load conditions No. 1-4 are achieved where thetarget is vibrated by 5 μm as one example according to this invention.However, the life improved under load conditions No. 3 and 4 may beconsidered still short. This invention, therefore, utilizes the factthat vibrating amplitude is varied easily. Further results of trialcalculations for vibration in 10 μm are supplemented in parentheses inFIG. 1. In this further case, even with load condition No. 4, the trialcalculations show a temperature of the colliding portion=2,217 K, andthe life=82,381 hours which is sufficiently long. That is, thisinvention readily realizes an X-ray intensity increased by three timesor more and a long life, thereby significantly improving workingthroughput.

Improvement regarding problem 3: “The tube is not darkened by minutefocusing.”

Load condition No. 5 in FIG. 1 shows a improvement example of thisinvention which is applied to the minute focal size needed in order tofollow the micro-fabrication of integrated circuits in the semiconductorfield today. In the load condition No. 5 in FIG. 1, the diameter ofelectronic collision is 0.1 μm. With the conventional fixed target, aninspection has to be conducted with X-ray intensity lowered to 0.032 W,i.e. a one-tenth of the load condition No. 1. When, despite this, theload is increased to 0.24 W as in load condition No. 5, the fixed targethas no life. However the vibration target with amplitude of 5 μm has thelife of 169 hours, which is an improvement to practice the condition No.5. This is no less than 20% longer than the life of 142 hours of theconventional fixed target under load condition No. 1. The X-rayintensity also is no less than 75% of that in load condition No. 1.

However, it may be felt that intensity is insufficient in theimprovement of load condition No. 5. Further results of trialcalculations for vibration in 10 μm with the same intensity as in loadcondition No. 1 (power of 0.32 W) are supplemented in parentheses inFIG. 1. The life is improved to 1,341 hours, which is sufficiently long.That is, according to this invention, the minute focusing does not makethe tube dark. Thus, a more detailed inspection can be conducted,without reducing working throughput, which is well fit for inspection ofadvanced minute semiconductors.

Improvement regarding problem 4: “Use is facilitated by only slightvariations in focal configuration.”

Conventionally, a microfocus X-ray tube do not keep high spatialresolution without a fine adjustment of the focal position even within alifetime. However, as noted in relation to the improvement made withrespect to problem 1, a life comparison under load condition No. 1 inFIG. 1 shows substantial improvements of this problem. The inventionprovides a life of 4.7×10²⁷ hours, which is regarded as an infinite lifeand an improvement over the 142 hours life of the fixed target. After ause period of 100,000 hours, the vibration target evaporates by athickness of only 2×10⁻¹⁹ μm. This poses no problem for the 1 μmdiameter of collision. Thus, a high spatial resolution is maintainedwithout adjustment, thereby the tube is easy to use.

As described above, problems No. 1-4 of the conventional example aresignificantly improved by this invention defined in claim 1. Theseimprovements have been described mainly in FIG. 1. In these trialcalculations, all the areas of electron collision due to the vibrationare defined as a linear track as shown in FIG. 4. FIG. 6 through FIG. 9illustrate other tracks of electron collision spot (claim 2).

FIG. 6 and FIG. 7 show examples that the target 9 is a arcuate shape inside view. These targets are swung accurately around a virtual circlecontaining arcuate target, and X-ray focus is on steady position.

FIG. 8 shows an example where the holder 11 is swung so that theelectron beam B describes an arcuate track on the surface of target 9.In this case, the holder 11 will be driven by a ring-like ultrasonicmotor to rotate back and forth to vibrate the target 9 arcuately asindicated by a two-dot chain line arrow. Instead of the ultrasonicmotor, an electrostatic motor will be used to apply vibration.

FIG. 9 shows an example where the holder 11 is vibratedtwo-dimensionally as indicated by two-dot chain line arrows, to providean electron collision area of 6 μm square. The holder 11 is vibratedright and left while vertically shifting at predetermined intervals sothat the electron beam B describes different sideways tracks asindicated by dotted lines in FIG. 9. Where each of the two sides of thetwo-dimensional vibration is 6 μm long and the diameter of electron beamcollision s=1 μm, the area is six times that of the linear track such asin FIG. 4. The temperature rise on the target surface derived fromequation (1) is 1/√6, which provides an advantage of further extendingthe life. In addition, the target surface is used fully and effectively.Conversely, the above embodiment minimizes the target size and theholder weight. As a result, the vibration power is a minimum to producea remarkable effect of minimizing the vibration unit. As an additionalexample, the target 9 is vibrated zigzag.

Next, examples of control by the vibration controller 25 is described.

The vibration controller 25 said in claim 3, controls vibrationamplitude Vw [μm] and vibration frequency Vf [Hz] to be optimal,according to a diameter of collision s [μm] of electron beam B, tubevoltage −Sv [V] or tube current Sa [A] set by the control unit 29.Alternatively, measuring a temperature adjacent the electron beamcollision spot controls the vibration.

A normal tube current Sa have a value proportional to a set value.Preferably, vibration control is based on a signal from an ammeter (notshown) measuring the target current directly.

The controls are effected such that the higher the temperature measuredadjacent the spot of electron beam collision, the smaller the collisiondiameter s, or the greater the electrical power, the greater thevibrating amplitude and frequency are.

As an example said in claim 4, the control of “vibration amplitude”,preferably, is based on the following equation (5):Vw=α·(Sv·Sa)/s  (5)

Where, for example, the amplitude is 5 μm which is effective for theimprovements relating to problems 1-4, coefficient α, preferably, is 5to 15. However, it is desirable to change coefficient α appropriatelyaccording to the heat conductivity K, load, life and so on of the targetmaterial.

However, when coefficient α=5, electrical power=1 W and diameter of thecollision s=5 μm, for example, the vibrating amplitude Vw is 1 μm. Thismeans that the electron beam B constantly strikes a target portion. Inorder to avoid this situation, it is desirable to determine from thefollowing condition formula after calculation of equation (4):

“Condition Formula”

When vibrating amplitude Vw<collision diameter s, vibration amplitude Vwis made equal to β·s. In this formula, coefficient β>1.

As an example set out in claim 5, the control of “vibration frequency”,preferably, is based on equation (6) shown hereunder.

When considering a thermal load occurring in a short time, it isnecessary to consider the moving speed ω [μm/sec]. This inventionassumes a moving speed ω due to vibration to be 2·Vw·Vf [μm/sec], andthe control of “vibration frequency”, preferably, is based on thefollowing equation (6):Vf=/(2·Vw)=ω·s/(2·α·Sv·Sa)  (6)

There is experimental data that temperature becomes 2,500° C. or less toprovide a long life when a rotational frequency is such as to move theelectron-colliding portion at 2 m/sec, for example. Based on this data,moving speed ω=2×10⁶ μm/sec. is considered sufficient. However, it isdesirable to change the moving speed appropriately according to the heatconductivity K, load, life and so on of the target. A sine wave ortriangular wave is used as a drive voltage waveform for vibration.

A supplementary description, about major differences from the rotatinganode type noted in problem 6, is following.

The greatest difference between the rotating anode type and thevibration type of this invention lies in the track length of theelectron beam. The rotating anode type uses a bearing or the like, andtherefore requires a disk target larger than the outer shape of thebearing. For example, even where the bearing has a minimum outer shapeof 10 mm, the target diameter is required to be about 11 mm. In thiscase, with the length of a track described by the electron beam being31.4 mm, the material being aluminum (density=2.7 g/cm³), and thethickness being 0.5 mm, the target is as heavy as 0.47 g. When thediameter of electron collision is about 1 μm as illustrated in thisinvention, a vibration amplitude of about 10 μm is sufficient. Theholder 11 have a size not exceeding 1×1 mm. The weight in this size isonly 0.0014 g. Thus, the invention achieves compactness, lightweight,and small driving power. The feature of little waste of the targetmaterial is also desirable from the viewpoint of resources andenvironment.

Examples of the vibration unit 23 in the above embodiment is describedin detail hereinafter by successively referring to FIGS. 10 through 19.

These examples include components said in claims 6-16 of this invention,which demonstrate characteristic effects in this invention. However,this invention is easily implemented with other mechanisms.

As set out in claim 6, a piezoelectric device is particularly suitablefor the vibration device contained in the claim 1.

The piezoelectric device is used as an actuator by the property of apiezoelectric material. A piezoelectric material applied an electricfield by electrodes is expanded and contracted corresponding to theelectric field direction and the polarization direction of the material.Materials for the piezoelectric device include polymers (e.g. copolymerof polyvinylidene fluoride and trifluoroethylene) and ceramics (e.g.having lead zirconate titanate [Pb(Zr,Ti)O³] as a main ingredient). Thecharacteristics of the piezoelectric actuator is the followings:

1. high precision controllability of micro displacement, 2. generatingstrong stress, 3. excellent high-speed response, 4. high energyconversion efficiency, and 5. no electromagnetic field occurring. As anactuator used in an increasing wide range of application, piezoelectricdevices are used for precision control of micro displacement inparticular, including precision positioning in semiconductor devicemanufacturing apparatus and STMs, adjustment of position, angle andfocal length of mirrors and lenses of cell-controlling micromanipulators or other optical equipment, and correction of errors inmachine tools. Piezoelectric devices are used also as ultrasonictransmitter and receiver elements. The displacement is varied fromseveral nanometers to several hundred micrometers, and responsefrequency from 0 Hz to several MHz.

Piezoelectric actuators are classified into two types, i.e. the lineardisplacement type that utilizes in-plane displacements and the curveddisplacement type that utilizes out-of-plane displacements.

Furthermore, the linear displacement type includes the single plate typeand laminate type. The single plate type, in many cases, is apiezoelectric plate polarized in the direction of thickness, for usingelastic displacements produced in the lateral direction by applying anelectric field parallel to the polarization P. Three types ofpiezoelectric deformations are produced, which are “verticaldeformation”, “lateral deformation” and “slip deformation”. The laminatetype is integrated with stacked piezoelectric plates and electrodes, andeach plate has a direction of reversed polarization from that of anadjacent plate. The laminate piezoelectric plates are electricallydriven parallel to one another to produce a displacement in a directionof lamination.

The curved displacement type includes a monomorph, unimorph, bimorph andmultimorph. The bimorph has two piezoelectric plates on both sides of ashim (thin metal plate) and is bended by applying an opposite electricfield to the pair plate. These have simple structures and a largedisplacement, but generate a weak force.

These piezoelectric devices displacements are generated by closedelectric fields between electrodes, and there is no magnetic field asdistinct from electromagnetic motors and the like. Thus, it is easy toshield an electric field so that piezoelectric devices prevent adverseinfluence on an electron beam, and the device can be disposed close tothe electron beam.

Even a small piezoelectric device generates a strong driving forceenough to vibrate the weight of a holder with ease. The vibrationapplying mechanism containing a piezoelectric device is small and can bemounted easily in the bore 7 with a diameter of 10 mm or less. Where, asin claim 13, the vibration applying mechanism is preferably mounted inthe bore 7, the target is disposed at a minimum distance to the electronlens. Since the aberration at a point of electron convergence is assmall as close to the electron lens, a minimum diameter of electronconvergence is obtained, also the X-ray focus is minimized. Furthermore,the small vibration unit allows the X-ray focus and inspection object tobe close each other, to increase photographic magnification, thereby toobtain X-ray fluoroscopic images of high spatial resolution. Further,with the micron-scale, high precision control and high speed features, apiezoelectric device is the best suited to the vibration applying deviceof this invention.

An example of vibration unit 23 using bimorphs among the abovepiezoelectric devices is described with reference to FIG. 10. FIG. 10Ashows a cross section and FIG. 10B shows a front view.

The vibration unit 23 shown in FIG. 10 includes a fitting 31 andpiezoelectric bimorphs 33. The fitting 31 is cylindrical, and isattached to the peripheral surface of the bore 7 of the end block 5. Thepiezoelectric bimorphs 33 are in plate form and extend from two, upperand lower positions of the fitting 31. The holder 11 forms aparallelogram attached at upper and lower ends thereof to distal ends ofthe bimorphs 33. These piezoelectric bimorphs 33 are arranged to bend inthe same direction, and an alternating voltage is applied to each. Then,as indicated by two-dot chain line arrows, these piezoelectric bimorphs33 swing and the target 9 is vibrated in directions parallel to thesurface, this vibration realizes a long life and high intensity X-raytube.

However, since the holder 11 forms a parallelogram, the target 9 issubject to shift in directions along the beam. Where, for example, thepiezoelectric bimorphs 33 are 5 mm long and the vibrating amplitude isonly 10 μm, the shape of piezoelectric bimorphs 33 is considered to beunchanged and substantially straight. A maximum shift in the directionof incidence of electron beam B is calculated at 5−√(5²−0.01²)=10 nm.However, even when the target 9 is shifted to this extent, the vibrationis sufficiently precise for the electron beam B having a normal X-rayfocal size of about 1 μm.

Even with a focal size of about 0.1 μm as an example of smaller size, asufficiently precise vibration is achieved by adopting a vibratingamplitude of 1 μm, in this case a maximum shift is calculated at5−√/(5²−0.001²)=0.1 nm. A ratio of the shift to the focal size in eachcase is 10 μm/1 μm=10 times, or 1 μm/100 nm=10 times. A large actualarea of electron collision is secured on the target 9 to disperse thegenerated heat, thereby to suppress a local temperature rise on thetarget surface due to the electron collision.

Another example of vibration unit 23 using bimorphs is described withreference to FIG. 11. FIG. 11A shows a cross section. FIG. 11B shows afront view. The track of the electron beam is schematically shown inFIG. 6.

In this example, vibration is applied so that, as shown in FIG. 6, thecollision spot describes an arcuate track in side view.

As in the construction described above, the vibration unit 23 includes afitting 31 and two piezoelectric bimorphs 33. The fitting 31 iscylindrical, and is attached to the peripheral surface of the bore 7 ofthe end block 5. The piezoelectric bimorphs 33 are in plate form andextend from right and left positions at the same height of the fitting31. The holder 11 has an arcuate section, and attached in verticallymiddle, right and left positions thereof to distal ends of the bimorphs33. These piezoelectric bimorphs 33 are arranged to bend in the samedirection, and an same alternating voltage is applied to each. Then, asindicated by a two-dot chain line arrow, these piezoelectric bimorphs 33swing and the holder 11 is vibrated in arcuate orbit whereby the target9 is vibrated in an arcuate orbit. In addition, the center of the arc ofthe holder 11 coincides with positions in which the piezoelectricbimorphs 33 are fixed to the fitting 31. Furthermore, the arc of theholder 11 has a radius corresponding to the length of piezoelectricbimorphs 33. Since the center of the arc lies on the optical axis of theelectron beam, the vibration does not shift the target in directionsalong the beam.

Further examples of vibration unit 23 are described with reference toFIGS. 12 and 13. FIGS. 12A and 13A show cross sections. FIGS. 12B and13B show front views.

These examples comprise piezoelectric devices 35 of the lineardisplacement type instead of piezoelectric bimorphs 33 described above.

The vibration unit 23 includes a fitting 31 and piezoelectric devices35. The fitting 31 is cylindrical, and is attached to the peripheralsurface of the bore 7 of the end block 5. The piezoelectric devices 35are prism-shaped and embedded in two, upper and lower inner peripheralpositions of the fitting 31. The holder 11 is plate-shaped and isattached at upper and lower ends thereof to inner walls of thepiezoelectric devices 35. The two piezoelectric devices 35 are embeddedto move minutely in the same direction together parallel to the surfaceof the target 9. When the piezoelectric devices 35 are driven, vibrationis applied to the target 9 parallel to the surface thereof as indicatedby two-dot chain line arrows. The piezoelectric devices 35 that undergolateral deformation or slip deformation are embedded in the fitting 31,and those that undergo vertical deformation are embedded at referencenumeral 35 b. Further, these piezoelectric devices will be the singleplate type or laminate type.

In the example shown in FIG. 12, it is unnecessary to consider a shiftin the direction of incidence of electron beam B as is necessary withthe piezoelectric bimorphs 33 in FIG. 10. Since the direction ofdisplacement is determined only by the characteristic of thepiezoelectric devices 35, vibrations is applied with increasedprecision.

As shown in FIG. 13, even a cantilever mode assures vibrations withsufficiently high precision, in reason that the holder 11 islightweight.

That is, this example provides only the lower one of the piezoelectricdevices 35 embedded in the two, upper and lower positions of the fitting31 in the preceding example. This produces the same effect as abovewhile simplifying the construction.

Next, two examples of vibration unit 23 relating to claim 7 is describedwith reference to FIGS. 14 and 15. FIGS. 14A and 15A show in sectionview. FIGS. 14B and 15B show front views.

The example shown in FIG. 14 is integrated together a plurality oflinear displacement type piezoelectric devices 35 of about 1 mm squareand several millimeters in height and attached to a fitting 31 to have asquare outer shape and compose a hollow space inside. The holder 11 isattached to the piezoelectric devices 35 so as to close the hollowspace. Each piezoelectric device 35 is operable to make a “slipdeformation”, and vibrate in directions parallel to the surface of thetarget 9 (vertically in FIG. 14A).

According to this construction, the piezoelectric devices 35 and holder11 are integrated to form a closed space. Consequently, the vacuumwindow 13 shown in FIG. 2 is no longer necessary. This simpleconstruction, allows the X-ray focus and inspection object to be closetogether to realize increased photographic magnification. Thus, theapparatus has a high resolution performance.

Although, in the above construction, a plurality of piezoelectricdevices 35 are used, a piezoelectric device 37 having a specialcylindrical shape is used as shown in FIG. 15.

This piezoelectric device 37 is manufactured by sinter-molding aferroelectric material, to have a cylindrical shape with an outsidediameter of about 5 mm and a length of about 5-20 mm. The piezoelectricdevice 37 is operable three-dimensionally. An example in which such apiezoelectric device 37 is applied is a three-dimensional scanner for ascanning probe microscope. The piezoelectric device 37 has a groundingelectrode mounted on an inner peripheral surface thereof, and fiveelectrodes X1, X2, Y1, Y2 and Z arranged on an outer peripheral surface.The electrodes X1 and X2 are opposed to each other along an X-axisextending perpendicular to the cylinder axis. The electrodes Y1 and Y2are opposed to each other along a Y-axis. The electrode Z is disposedannularly on an upper outer peripheral surface around a Z-axis extendingalong the cylinder axis.

This piezoelectric device 37 is extendible when a positive voltage isapplied, and contractible when a negative voltage is applied, to theelectrodes disposed on the outer peripheral surface opposite thegrounding electrode. The piezoelectric device 37 is attached to thefitting 31. When the portion including the electrodes X1, X2, Y1 and Y2is attached to the fitting 31 and when a voltage of opposite polaritiesis applied to the electrodes X1 and X2 opposed to each other, thepiezoelectric device 37 operates as shown in FIG. 15B. That is, theportion of electrode X1 extends while the portion of electrode X2contracts, whereby the whole device 37 bends to displace the portion ofelectrode Z in the X direction.

An amount of displacement at the distal end is determined by thecylinder length and the voltage applied. A scan signal applied isprovided for scans from 1 nm to several tens of micrometers by a voltageof several volts to 200V.

By bonding the holder 11 to the top of this piezoelectric device 37, thesame effect is produced as the construction shown in FIG. 14. Moreover,since the target can be moved in the Z direction, also interlocking thepiezoelectric device and the electron lens move the X-ray focusposition. This provides an advantage of a fine adjustment ofphotographic magnification without moving a inspection object. Applyinga voltage to the electrode Z cause a very minute extension orcontraction in the order of 10 nm/V in the Z direction.

As set out in claim 8, the vibration unit of this invention preferablycontains some flexures as support elements thereof. Where minutedisplacements of 1 mm or less is required in this invention, flexures isthe plastic deformation element that is free from slips, staticfriction, kinetic friction and back crash under the severe environments.Flexures have various kinds, which are called a spring, a coil spring,spring plate and other. These flexures are the best suited support partsfor this invention under a high vacuum, high temperature and high speed,because lubricant (grease) is unnecessary like the steel ball bearing.Flexures have a further advantage of being small, simple, low cost andhighly precise.

Examples using flexures is described in order, referring to FIGS. 16through 18. FIG. 16A shows a cross section. FIG. 16B shows a front view.FIG. 17 shows a front view. FIG. 18 shows a cross section.

The construction shown in FIG. 16 is similar to the construction shownin FIG. 12. The difference is that the flexures 39 is attached betweenthe fitting 31 and the holder 11. The portion, flexures 39 and holder11, flexures 39 and fitting 31, is joined by adhesive or welding thatpreferably provide high heat conductivity.

The material for flexures 39, preferably, is ceramic or metal from theviewpoint of heat conductivity, and further preferably, phosphor bronzeor beryllium copper which is a material for springs, from the viewpointof durability. Furthermore, it is desirable to cut off flexures 39 froma thick metal plate by electrical discharge machining from the viewpointof processing accuracy (claim 9).

The flexures 39 release the heat of the target 9 through the holder 11,and suppress a deflection of the target 9 in directions along theelectron beam. Thus, a vibration deviation of the X-ray focus issuppressed.

Of course, the flexures 39 will be attached on the other mechanism;FIGS. 10 through 15, contained the piezoelectric devices.

FIG. 17 shows a construction similar to FIG. 16. The difference is thatthe flexures 39 and fitting 31 are replaced here with the fitting 50integrated flexure portions 51; U-shaped hinge. The holder 11 of thetarget 9 will be connected by a thermally conductive adhesive orwelding. However, FIG. 17 shows an integrated mold including the holder11.

As set out in claim 14, the flexure portions 51 are thin in thedirection of vibration of the target 9 and thick in the directionperpendicular to the direction of vibration. These flexure portions 51characterized by high aspect ratio will be formed by electricaldischarge machining, for example. Another shapes are conceivable, suchas a simple plate or radial shapes. Such flexures of high aspect ratiois driven by a small force in the direction of vibration, but aredifficult to move in the direction perpendicular to the direction ofvibration. Thus, the flexures enable highly precise vibrations of thetarget 9 without deflection in directions along the electron beam. Theflexures are suitable for a element of a vibration applying mechanism ofan X-ray tube having a submicron X-ray focus of several microns or less.The integrated mold formation is desirable also from a viewpoint ofassembling accuracy.

FIG. 18 is a cross section showing a different construction of thevibration unit 23 using flexures.

A vacuum window (13) acts also as a holder 11A, and has flexures 39 aformed peripherally thereof. Drive devices 36 are connected to theholder 11A through connecting plates 41. The holder 11A is cut from acylindrical metal block by electrical discharge machining, for example.The holder 11A will be formed identically with the connecting plates 41.

Since vibration is applied to the target 9 through the holder 11, thetarget 9 is vacuum-sealed by the flexures 39 a capable of absorbingvibration. Thus, the vacuum window (13) of FIG. 2 is dispensed with, tominimize a distance between the X-ray focus and inspection object, andgeometrically increase resolution. The portions of flexures 39 a will beformed of elastic elements, such as rubber elements or bellows (claim10).

Next, the construction set out in claim 11 is described.

Improvement regarding problem 5: “Unnecessary absorption of X-rays bythe target is eliminated by thinning the target.”

As described in Problem 5 hereinbefore, the conventional target is thickand unnecessarily absorbs X-rays. In this invention, vibrating causes anextended life even to the thin target, and therefore a transmittingX-ray dose increases.

For example, electrons with an energy of 40 keV accelerated in time of40 kV tube voltage collide with the tungsten target, and penetrate amaximum depth of 2.6 μm to the target. Since the target has an extendedlife in this invention, the target have a thickness corresponding to themaximum electron penetration depth of 2.6 μm. This eliminates the 20%X-ray absorption by the 2.4 μm tungsten conventionally added as anextra. Thus, the target according to this invention has 1.2 times theworking throughput of the conventional 5 μm target. The effect isparticularly outstanding at low energy X-ray with a large proportion ofabsorption.

When electrons accelerated by E[kV] collide with a target of densityρ[g/cm³], a maximum electron penetrating depth R[μm] is derived from thefollowing equation (4):R=0.0021 (E ²/ρ)  (4)

A target thickness for maximizing X-ray generation corresponds to themaximum penetrate depth R in time of acceleration voltage E[kV]. Thus,the optimum target thickness is adopted from the equation (4).

Although the target thickness is not necessarily limited to R,generally, this invention effect is expected roughly within twice R.This is well suited particularly where easily absorbable soft X-rays aregenerated.

When a collision diameter s [μm] is less than the micron scale and anaccreting voltage is over around 40 keV, a target thickness t [μm]substantially corresponding to the collision diameter s is desirablefrom the viewpoint of a minute X-ray focal size (claim 15).

Next, the construction set out in claim 12 is described.

When the electron beam power is low, the vibration controller 25displaces the target as follows.

When the electron beam power is low, the vibration unit preferablydisplaces the target at every several months or several weeks, forexample, to change positions of the electron collision spot. In thiscase, vibration may or may not be applied to the target in eachposition. Such displacement move the colliding spot of electron beam Bto a different positions in few seconds, and dispenses with evacuatingtimes as required with the fixed type. The quick changing avoidsdeterioration in working throughput or time.

This invention is not limited to the foregoing embodiments and will bemodified as follows or more:

(1) The drive element of the vibration unit 23 is an electrostrictiondevice, an electrostatic actuator, or a magnetostrictive device.Further, an electromagnetic motor or a solenoid will be utilizable, whenthese are disposed remote from the electron beam, or with a magneticshield inserted. Such a construction provides a significant advantage ofextending life, but although not attaining compactness or highresolution.

(2) The flexures of the vibration unit 23 will be replaced with wiresprings, metal gauzes, slip bearings, ceramic ball bearings, elasticmetal elements, for example.

(3) All the examples described above relate to a transmission X-raygenerating apparatus 1. This invention is applicable also to areflection X-ray generating apparatus. FIG. 19 is a cross sectionshowing a target and adjacent components of a reflection X-raygenerating apparatus 1A.

This reflection X-ray generating apparatus 1A according to thisinvention includes a support base 43 for locating a holder 11 having atarget 9 at an angle to a direction of electron beam B. The support base43 has a coupling rod 45 attached to a center forward position thereofthrough a piezoelectric device 35. The holder 11 is attached to theforward end of the coupling rod 45. Flexible connecting plates 47interconnect side surfaces of the holder 11 and side surfaces of thesupport base 43.

When driven, the piezoelectric device 35 applies vibration to the target9 in directions parallel to the surface thereof. Thus, with such areflection X-ray generating apparatus 1A, as with the transmission X-raygenerating apparatus 1, the invention produces a similar thermal effectto realize a long life and high X-ray intensity (claim 16).

INDUSTRIAL UTILITY

As described above, this invention is suited for an X-ray generatingapparatus with high resolution and compactness, for extending the lifeof a target, increasing the operating ratio of the apparatus, extendinga time of continuously generating X-rays, and improving X-ray intensity,which are achieved by vibrating the target and enlarging an effectiveelectron-colliding area.

1. An apparatus for generating X-rays by irradiating a target with anelectron beam, comprising: an electron gun operative for emittingelectrons; an electron lens having a bore extending therethrough forreceiving and converging the emitted electrons; vibration applying meansfor vibrating said target in directions parallel to a surface thereof,the vibration applying means disposed within the bore and connected tothe electron lens; a holder connected to the vibration applying meansand operative to hold the target within or adjacent the bore; and avacuum vessel operative for containing the electron gun, the electronlens, the vibration applying means and the target in a vacuum.
 2. Anapparatus as defined in claim 1, wherein said vibration applying meansincludes a piezoelectric device.
 3. An apparatus as defined in claim 1,wherein said vibration applying means is arranged to vibrate said targetso that said electron beam has a colliding spot describing, on saidtarget, one of a linear track, a circular track, and a two-dimensionalshape including zigzag and rectangular shapes.
 4. An apparatus asdefined in claim 1, further comprising a vibration controller forcontrolling said vibration applying means based on one of a voltage, acurrent, an electron beam diameter, and a temperature measured adjacenta spot of electron beam collision.
 5. An apparatus as defined in claim4, wherein said vibration controller is arranged to control a magnitudeof vibration amplitude, the magnitude of the vibration amplitude beingmore than the electron beam diameter.
 6. An apparatus as defined inclaim 4, wherein said vibration controller is arranged to make thevibration frequency variable.
 7. An apparatus as defined in claim 2,wherein said piezoelectric device is integrated with said holder havingsaid target to define a closed space.
 8. An apparatus as defined inclaim 7, further comprising flexures for attaching and supporting saidholder.
 9. An apparatus as defined in claim 8, wherein said flexures aremade by electrical discharge machining.
 10. An apparatus as defined inclaim 1, further comprising rubber elements or flexures to provide avacuum seal.
 11. An apparatus as defined in claim 1, wherein said targethas a thickness up to twice the depth of electrons penetrationcalculated from a voltage and said target material.
 12. An apparatus asdefined in claim 1, wherein said vibration applying means is arranged todisplace said target.
 13. An apparatus as defined in claim 1, whereinsaid vibration applying means is disposed in a bore in which said targetis located.
 14. An apparatus as defined in claim 8, wherein saidflexures are shaped thin in a direction of vibration of said target, andthick in a direction perpendicular to the direction of vibration.
 15. Anapparatus as defined in claim 1, wherein said target has a thicknesscorresponding to a diameter of said electron beam colliding with saidtarget.
 16. An apparatus as defined in claim 1, wherein said target isdisposed at an angle to said electron beam.